Investigation of slow surface states on a real germanium surface by the optical excitation technique

The slow states (SS) charging ΔQ_s under the action of light quanta of different energy (2 ? hv ? 4.6 eV) has been investigated on a real germanium surface. The considerable influence on the optical SS charging of the preparation method as well as of adsorption-desorption processes has been revealed. On the basis of the spectral dependencies ΔQ_s(hv) the conclusion has been made about the existence of the adsorption-sensitive system of “fluctuation” electron states near the edges of energy bands of the oxide layer. The photocharging method has been shown the construction of the whole energy scheme of the semiconductor-dielectric heterojunction (including the band gap of the dielectric layer). The possible origin of the deep traps in oxide layer which are responsible for the optical charging of a real germanium surface, has been discussed.     

Averaged local field intensities in composite films

The photophysical properties of molecules adsorbed in composite films (e.g. surface island films) depend on the local electromagnetic field within the film. The ratio between the average field intensity 〈|E|²〉 in the film and the intensity |E_I|² associated with the incident field is a measure of the electromagnetic contribution to the surface influence on molecular photophysical phenomena. This ratio depends on the film composition and morphology, on the dielectric properties of the pure components making the film and on the frequency, direction and polarization of the incident radiation. Calculations of this ratio as a function of these parameters for several models of composite films are presented. Image interactions and retardation effects as well as radiative damping and finite size contributions to the dielectric response of the films are taken into account. In addition, an estimate of the field inhomogeneity within the film is obtained by calculating also the ratio 〈|E|²〉_shell/|E_I|² associated with the field in thin shells surrounding the dielectric particles which constitute the film.     

Optical surface polaritons of TM type at the nonlinear semiconductor–nanocomposite interface

TM-polarized optical surface polaritons in a nonlinear semiconductor–nanocomposite guiding structure have been considered. The nanocomposite consists of alternating layers of bismuth-containing garnet ferrite (BIG, Lu_{3 – x}Bi_xFe_{5 – y}Ga_yO₁₂) and gallium–gadolinium garnet (Gd₃Ga₅O₁₂), and the semiconductor (n-InSb) has a cubic nonlinearity and is characterized by two components of the nonlinear susceptibility tensor. With allowance for the anisotropy of the optical properties of the nanocomposite, caused by the magnetization of the BIG layers, the dispersion relation has been obtained and analyzed and its solutions are shown to split into two pairs of high- and low-frequency branches. The influence of the electric field at the interface on the wave characteristics and the existence domains of nonlinear surface TM polaritons has been studied. By solving the inverse problem of finding the profile of the longitudinal electric component of the surface polariton, it has been found that the nonlinearity gives rise to soliton-like wave fields.     

Electromagnetic wave scattering from two interacting small spherical particles. Influence of their optical constants, ε and μ

In this research, the influence of the optical constants ε and μ on the scattering patterns of a system consisting of two interacting Rayleigh particles is analyzed. We study specific scattering configurations in which the particles are separated by a fixed distance and where the connecting axis has fixed orientations with respect to the incident electromagnetic field. Multiple scattering effects and how they are affected by the optical properties of the particles are considered.     

New resonance-polariton Bose-quasiparticles enhances optical transmission into nanoholes in metal films

We argue the existence of fundamental particles in nature, neutral Light-Particles with spin 1, and rest mass m=1.8⋅¹⁰−4me, in addition to electrons, neutrons and protons. We call these particles Light Bosons because they create the electromagnetic field which represents Planck's gas of massless photons together with a gas of Light-Particles in the condensate. In this respect, the condensed Light-Particles, having no magnetic field, represent the constant electric field. In this context, we predict an existence of polariton-plasmon Bose-quasiparticles with effective masses ml≈¹⁰−6me and mr=0.5me, which are induced by interaction of the plasmon field and the resonance Frölich-Schafroth charged bosons with electromagnetic wave in metal. Also, we prove that the enhancement optical transmission into nanoholes in metal films and Surface Enhanced Raman Spectroscopy are provided by a new resonance-polariton Bose-quasiparticles but not model of surface plasmon-polariton. In this Letter, the quantization Fresnel's equations is presented which confirms that Light-Particles in the condensate are concentrated near on the wall of grooves in metallic grating and, in turn, represent as the constant electric field which provides the launching of the surface Frölich-Schafroth bosons on the surface metal holes.     

Effects of applied electric and magnetic fields on the nonlinear optical rectification and second-harmonic generation in a graded quantum well under intense laser field

In this present study, the effects of electric and magnetic fields on the nonlinear optical rectification and second-harmonic generation in a graded quantum well under intense laser field have been investigated theoretically. The energy eigenvalues and their corresponding eigenfunctions are obtained by solving Schrödinger equation within the framework of effective mass approximation. The analytic expressions for the optical properties are calculated by the compact-density-matrix approach and iterative method. The numerical results are presented for a typical GaAs/Ga_1?x Al _x As quantum well. The results show that the nonlinear optical rectification and second-harmonic generation coefficients are considerably affected by the electromagnetic fields and intense laser field.     

Magnetization of epitaxial MnGe alloys on Ge(1 1 1) substrates

The structural and magnetic properties of an Mn rich solid phase epitaxy Mn_xGe_1−x alloy grown on a clean 2 × 8-Ge(1 1 1) surface, with a Curie temperature of about 300 K are investigated. Magneto-optical  Kerr effect infers the existence of in-plane easy magnetization direction. We describe the epitaxial registry condition, the room temperature—zero field magnetic structure observed by magnetic force microscopy and the magneto-optical properties. The observations are consistent with the formation of epitaxial Mn₅Ge₃ alloy, with a modulated magnetic structure characterized by asymmetric 180° Bloch walls consisting of a vortex-like configuration of the local magnetization.     

Electric field induced exciton binding energy and its non-linear optical properties in a narrow InSb/InGaxSb1−x quantum dot

The effect of electric field on the binding energy, interband emission energy and the non-linear optical properties of exciton as a function of dot radius in an InSb/InGa_xSb_1?x quantum dot are investigated. Numerical calculations are carried out using single band effective mass approximation variationally to compute the exciton binding energy and optical properties are obtained using the compact density matrix approach. The dependence of the nonlinear optical processes on the dot sizes is investigated for various electric field strength. The linear, third order non-linear optical absorption coefficients, susceptibility values and the refractive index changes of electric field induced exciton as a function of photon energy are obtained. It is found that electric field and the geometrical confinement have great influence on the optical properties of dots.     

Influence of superalkali oxides on structural,electrical and optical properties of C24 fullerene nanocluster: A theoretical study

The influence of superalkali oxides on structural, electrical and the optical properties of 6-5 isomer of C₂₄ fullerene nanocluster was investigate using DFT calculations. It was shown that the structure of fullerene was drastically affected by adsorption of M₃O on its surface and as a result of all bonding lengths, was subsequently changed. DOS results showed that the E_g of C₂₄ in presence of M₃O (M=Li, Na, K) was fairly changed so that the electronic properties and electrical conductance of C₂₄ were not affected by M₃O adsorption. Based on the results of NBO calculations, specifically on MEP surfaces and HOMO-LUMO distribution, the charge was transferred from M₃O to C₂₄ fullerene; therefore, the M₃O is the positive head of complexes and in an electric field it is oriented toward the negative pole. The calculated polarisability(α) and the first hyperpolarisability(β₀) values indicated that the adsorption of M₃O on exterior surface of C₂₄ fullerene has had a drastic effect on the optical response of C₂₄ fullerene and caused a large increase in α and β₀ values. TD-DFT calculations were also carried out in order to confirm the β₀ values. This work theoretically devises novel C₂₄-based nanostructures via interaction with Superalkali Oxides in order to promote, their potential applications in electronic devices and high-performance NLO materials.     

Electrical and optical characteristics of porous silicon impregnated with LaF3 by a novel chemical bath technique

The structural, electrical and optical characteristics of porous silicon (PS) due to the impregnation of LaF₃ into PS by a novel chemical-bath deposition (CBD) technique have been investigated in this article. Without removing the PS from the anodization chamber the impregnation with LaF₃ has been done by reacting LaCl₃ with HF in the same chamber at room temperature. The impregnation of LaF₃ was confirmed by the SEM on the cross-section of the LaF₃/PS/Si system and EDX. The modification of PS surface by LaF₃ had direct influence on the electrical and optical properties of PS. Electrical properties of Ag/LaF₃/PS/p-Si/Ag structures were studied through the current-voltage (I-V) and capacitance-voltage (C-V) characteristics. Formation of metal-insulator-semiconductor (MIS) diode was evident whose forward current increased with annealing. A diode factor of about 2.4 has been obtained for the annealed heterostructure indicating a high density of trap states. The C⁻²-V curves of all samples showed negative flat band voltage of around −2 V confirming a large number of fixed positive charges in the LaF₃. The photoluminescence (PL) intensity of the LaF₃-impregnated PS showed aging for the as-deposited samples, but when annealed PS structure recovered the PL intensity. Experimental results show that the optimized chemical bath passivation process for the LaF₃ on porous silicon could enable the porous silicon to be an important material for photonic application.     

Coherent transients observed by Stark switching in ruby having narrow optical hole burning

By using the Stark switching technique we observed new coherent transient signals in ruby at low laser intensities, which are unique in solids where T₁ ? T₂. It was confirmed that the existence of narrow optical hole burning before the switching plays an essential role in giving rise to the signals. The effects of the multiple optical hole burning were also observed in the time domain. Assuming the existence of the holes, the signals can well be interpreted by linear system analysis.     

Synthesis of CdZnO thin film as a potential candidate for optical switches

Cadmium doped zinc oxide thin films have been prepared using a thermal decomposition technique. The influence of Cd as a doping agent on the structure, optical and nonlinear optical properties was carefully investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) and a UV-vis spectrophotometer. A deep correlation has been found between the surface roughness and the optical properties. The roughness is found to deteriorate the nonlinear response, such that the highest nonlinear susceptibility χ⁽³⁾ is obtained for the smoothest layer. The third-order nonlinear susceptibility χ⁽³⁾ has been calculated using the Frumer model, and is estimated to be 3.37×10⁻¹⁰ esu. The dispersion of the refractive index of the prepared thin film is shown to follow the single electronic oscillator model. From the model, the values of oscillator strength (E_d), oscillator energy (E_o) and dielectric constant (ε_∞) have been determined. The conductivity has been measured as a function of the energy of the photons, revealing marginal change at energies below 3.15 eV, while above this value there is a large increase in the conductivity. This suggests that CdZnO is a potential candidate for applications in optical devices such as optical limiter and optical switching.     

X-ray diffraction and optical studies of CdS1−x Sex solid solutions with stacking faults

The existence in CdS_1?x Se_x crystals (with x=0.10–0.50) of crystalline regions with stacking faults (SF) was first demonstrated by x-ray diffraction and optical methods. X-ray diffraction studies showed SF to be present in all the samples investigated, but in different concentrations. The effect of SF present in CdS_1?x Se_x solid solutions of different compositions on their exciton reflectance and photoluminescence (PL) spectra has been studied. Crystals with high SF concentrations were found to exhibit new exciton bands, which are manifested in reflectance and PL spectra. In the CdS_1?x Se_x phase with SF, resonant emission due to free excitons and the corresponding phonon replicas have been observed. The effect of reabsorption, which can bring about a change in the zero-phonon emission line shape (doublet formation), as well as affect the intensity ratios of the zero-phonon line to the phonon replicas, has been analyzed. It is pointed out that the variation with temperature of the shape of the SF-induced PL exciton line indicates its complex structure, with the constituents of this structure varying in intensity with increasing temperature.     

Time-resolved nonlinear surface plasmon optics

A new surface-sensitive method of time-resolved optical studies is proposed. The method consists in the independent excitation of several surface electromagnetic waves (SEW) by two laser femtosecond pulse beams with varied time delay Δτ and distance Δr between corresponding excitation regions on the surface. To fulfill the phase-matching condition for plasmon-photon coupling, metal grating is used. Due to nonlinear plasmon interaction, the optical radiation with ω₁ + ω₂ and 2ω₁ ? ω₂ (where ω₁, ω₂ are corresponding laser beam frequencies) is generated. The intensity of this nonlinear response versus Δτ and Δr are studied. The direct measurements of the SEW temporal properties are presented. Experiments of this type are important for the development of femtosecond surface plasmon optics.     

On the mechanism of electromagnetic microwave absorption in superfluid helium

In experiments on electromagnetic (EM) wave absorption in the microwave range in superfluid (SF) helium [1?C3], a narrow EM field absorption line with a width on the order of (20?C200) kHz was observed against the background of a wide absorption band with a width of 30?C40 GHz at frequencies f ₀ ?? 110?C180 GHz corresponding to the roton gap energy ?? _r (T) in the temperature range 1.4?C2.2 K. Using the so-called flexoelectric mechanism of polarization of helium atoms (⁴He) in the presence of density gradients in SF helium (HeII), we show that nonresonance microwave absorption in the frequency range 170?C200 GHz can be due to the existence of time-varying local density gradients produced by roton excitations in the bulk HeII. The absorption bandwidth is determined by the roton-roton scattering time in an equilibrium Boltzmann gas of rotons, which is t _r-r ?? 3.4 × 10^?11 s at T = 1.4 K and decreases upon heating. We propose that the anomalously narrow microwave resonance absorption line in HeII at the roton frequency f ₀(T) = ??r(T)/2??? appears due to the following two factors: (i) the discrete structure of the spectrum of the surface EM resonator modes in the form of a periodic sequence of narrow peaks and (ii) the presence of a stationary dipole layer in HeII near the resonator surface, which forms due to polarization of ⁴He atoms under the action of the density gradient associated with the vanishing of the density of the SF component at the solid wall. For this reason, the relaxation of nonequilibrium rotons generated in such a surface dipole layer is strongly suppressed, and the shape and width of the microwave resonance absorption line are determined by the roton density of states, which has a sharp peak at the edge of the roton gap in the case of weak dissipation. The effective dipole moments of rotons in the dipole layer can be directed either along or across the normal to the resonator surface, which explains the experimentally observed symmetric doublet splitting of the resonance absorption line in an external dc electric field perpendicular to the resonator surface. We show that negative absorption (induced emission) of EM field quanta observed after triggering a Kapitza ??heat gun?? occurs when the occupation numbers for roton states due to ??pumping?? of rotons exceed the occupation numbers of EM field photons in the resonator.     

The optical properties of expanded liquid cesium

Abstract

The density dependence of the optical properties of expanded cesium has been investigated by means of reflectivity measurements in the range 0.5 – 4.0 eV. The dielectric functions ?₁ (ω) and ?₂(ω) have been derived by means of a modified Kramers-Kronig analysis. The data give useful information about the variation of the electronic structure of liquid cesium with large changes in density, large enough to reveal the existence of precursors of the metal-nonmental transition.     

Influence of annealing temperature on microstructure and optical properties of sol-gel derived tungsten oxide films

Tungsten oxide (WO₃) thin films have been extensively studied for their interesting physical properties and a variety of potential applications in electrochromic devices. In order to explore the possibility of using these in electrochromic devices, a preliminary and thorough study of the optical properties of the host materials is an important step. Based on this, the influence of annealing temperature on the structural, surface morphological, optical and electrochromic properties has been investigated in the present work. The host material, WO₃ films, has been prepared from an ethanolic acetylated peroxotungstic acid sol containing 5 wt.% oxalic acid dehydrate (OAD) by sol-gel technique. The monoclinic structure and textured nature change of the films with the temperature increasing have been investigated by X-ray diffraction analysis. The surface morphology evolution of the films has been characterized by SEM. The shift in absorption edge towards the higher wavelength region observed from optical studies may be due to the electron scattering effects and the optical band filling effect that reveals the crystallization of the film. The amorphous film shows better optical modulation (ΔT = 76.9% at λ = 610 nm), fast color-bleach kinetics (t_c ∼ 4 s and t_b ∼ 9 s) and good reversibility (Q_b/Q_c = 90%), thereby rendering it suitable for smart window applications.     

Properties of superfine superconducting filaments embedded in normal matrix

The size-, surface- and proximity-effects are combined for calculating the critical temperature and some unusual electromagnetic properties of superfine superconducting NbTi filaments (i.e. with a diameterd _s < 0·1 μm) in the neighbourhood of normal metal (Cu). It is shown that the existence of the one-dimensional flux line lattice is responsible for the extremely large values of the critical current densities in low magnetic fields (B < 0·2B _c2,B _c2 — the upper critical magnetic field), for the maximum of the volume pinning force at low magnetic fields (B _m ≈ 0·1B _c2, comparing withB _m≈ 0·5B _c2 in usual macroscopic multifilamentary NbTi superconductors), as well as for the peculiar minimum of the magnetization after reversing the magnetic field direction. The possible role of the frozen-in flux lines in creating flux lines of opposite direction is sketched. It is proposed that for practical purposes the superconducting filaments should be coated with electrically insulating material (e.g. cupro-nickel or dielectric) to destroy the electro-magnetic coupling between the filaments and to ensure low ac losses in the composite.     

Lattice dynamics of ZnSe x S1 − x semiconductor crystals

Recently ultrabroadband infrared solid state lasers based on a new vibronic material Cr²⁺:ZnSe _x S_1–x were demonstrated [1–3]. Cr²⁺ ion substitutes the metal ion (tetrahedral sites), the crystal field of the solid solution is responsible for large inhomogeneous broadening of Cr²⁺ electron states. The crystal field can be reconstructed by investigation of lattice dynamics — optical phonon parameters and dielectric function in IR. We paid special attention to investigation of vibrational and infrared spectroscopic properties of ZnSe _x S_{1 ? x} crystals. A very interesting and somewhat unexpected result of these studies was the existence in the crystals of effective S-Se dipoles, which generate an additional deep dynamically charged level in the forbidden gap of the semiconductors. The results of the first-principles calculations of both the phonon structure and the electron localization in ZnSe _x S_1–x crystals as well as acceptor levels in Cr²⁺: ZnSe crystal are discussed.     

Optical polarized properties related to the oxygen vacancy in the CaMoO4 crystal

The electronic structures, dielectric functions and absorption spectra for the CaMoO₄ (CMO) crystal with and without oxygen vacancy V_O²⁺ have been calculated using the CASTEP code with the lattice structure optimized. The calculated results indicate that the optical properties of the CMO crystal show anisotropy and its optical symmetry coincides with the lattice structure geometry of the CMO crystal. The calculated absorption spectra indicate that the perfect CMO crystal does not display absorption band in the visible and near-ultraviolet range. However, in this range, the absorption spectra of the CMO crystal containing V_O²⁺ exhibit one peak at about 1.84 eV (673 nm). It predicates that the 680 nm absorption band is related to the existence of V_O²⁺ in the CMO crystal.